Eukaryotic chromatin is composed of macromolecular complexes called nucleosomes. A nucleosome has 147 base pairs of DNA wrapped around a protein octamer having two subunits of each of histone protein H2A, H2B, H3, and H4. Histone proteins are subject to post-translational modifications which in turn affect chromatin structure and gene expression. One type of post-translational modification found on histones is methylation of lysine and arginine residues. Histone methylation plays a critical role in the regulation of gene expression in eukaryotes. Methylation affects chromatin structure and has been linked to both activation and repression of transcription (Zhang and Reinberg, Genes Dev. 15:2343-2360, 2001). Enzymes that catalyze attachment and removal of methyl groups from histones are implicated in gene silencing, embryonic development, cell proliferation, and other processes.